1. Field of the Invention
The invention relates to the field of electronic circuits, and in particular, to an efficient, low noise fractional charge pump.
2. Related Art
Most portable electronic devices contain digital and analog circuits operating at 2.5 Volts or below. However, the battery power used in such devices generally provides a supply voltage that is above the operating voltage of these devices (typically around 3.6 V). For example, a modern rechargeable lithium ion or lithium polymer battery is typically rated to have a nominal output voltage of 3.7 V, but may actually provide a voltage in the range of 2.7 to 4.2 V, depending on the charge state of the battery.
This variability in battery supply voltage necessitates circuitry to step down the supply voltage to the acceptable level. One of the common schemes is to use a charge pump with multiple capacitors. A charge pump can have 2 capacitors equally dividing the battery voltage.
An implementation of such type of charge pump is known as a “1/2×” charge pump. FIGS. 1A and 1B are schematic diagrams of a conventional 1/2× charge pump 100, which receives an input voltage V_IN1 and provides a reduced output voltage V_OUT1 to a load D140. Charge pump 100 includes an input terminal 101, charging capacitors C110 and C120, a storage capacitor C130, and an output terminal 102. While not shown for clarity, charge pump 100 also includes interconnect circuitry for connecting capacitors C110 and C120 in the configurations shown in FIGS. 1A and 1B.
Charge pump 100 operates by switching between the two phases of operation shown in FIGS. 1A and 1B. In FIG. 1A, a charging phase is shown, in which capacitors C110 and C120 are serially connected between input terminal 101 and ground, while capacitor C130 is connected between ground and output terminal 102 (load D140 is always connected between output terminal 102 and ground). During this charging phase, capacitors C110 and C120 are charged by input voltage V_IN1 to voltages V11 and V12. Under steady state conditions, capacitors C110 and C120 will both be charged to half of input voltage V_IN1 during this charging phase. Meanwhile, a voltage V13 stored on capacitor C130 is provided as output voltage V_OUT1 for driving load D140.
Then, in a discharging phase shown in FIG. 1B, capacitors C110 and C120 are connected in parallel between input terminal 101 and output terminal 102. Specifically, the positive plate (marked with a triangular indicator) of capacitor C110 is connected to input terminal 101, while the negative plate (unmarked) of capacitor C110 is connected to output terminal 102. Likewise, during the discharging phase, the positive plate (marked) of capacitor C120 is connected to the input terminal 101, while the negative plate (unmarked) of capacitor C120 is connected to output terminal 102.
Because capacitors C110 and C120 are inverted and connected in parallel after input terminal 101, the output voltage V_OUT1 provided during the discharging phase shown in FIG. 1B is equal to the difference of input voltage V_IN1 and the average of voltages V11 and V12 on capacitors C110 and C120, respectively. As described above with respect to FIG. 1A, both capacitors C110 and C120 are charged to half of input voltage V_IN1 during the charging phase. Therefore, the output voltage V_OUT1 provided during the discharging phase is simply equal to one half of input voltage V_IN1 (i.e., 0.5*V_IN1).
Therefore, the output voltage range of 1/2× charge pump 100 is between 1.35 V and 2.1 V when provided with a lithium ion battery voltage (i.e., 2.7 V to 4.2 V) as in input voltage.
As portable devices become increasingly advanced while at the same time shrinking in size, power efficiencies must continually be improved. While 1/2× charge pump 100 can provide a reduced supply voltage of half the battery voltage, the battery voltage can vary significantly, thereby resulting in significant variation in the reduced supply voltage. For example, the output voltage range of 1/2× charge pump 100 is between 1.35 V and 2.1 V when provided with a nominal 3.7 Volt lithium ion battery having a voltage range of 2.7 V to 4.2 V as an input voltage. In this case, the desired nominal output voltage is about 1.85 V. Thus, the output voltage provided by 1/2× charge pump 100 may be significantly below the desired nominal output voltage. In this case, the available battery charge is small and the efficiency is also small. For this reason, 1/2× charge pump 100 is not ideally suited for use in all applications.
It would therefore be desirable to have a charge pump capable of applying a multiplication factor greater than 1/2× and less than 1× to an input voltage. It would also be desirable to have a system and method for stepping down a supply voltage that maximizes power efficiency while minimizing die area requirements.